This invention relates to a circuit arrangenment for A.C. operation of high-pressure gas discharge lamps comprising a current limiter arranged between the lamp and the mains alternating voltage source and a high-frequency oscillator supplied with direct current and producing a high-frequency current through the lamp superimposed on the mains alternating lamp current. The oscillator comprises a high-frequency transformer and a transistor which is connected in series with the primary of the transformer and can be periodically switched on and off, while a secondary of the transformer is connected in series with the lamp. As a current limiter, use may be made of an ohmic resistor, a choke coil or an electronic ballast unit.
A problem in the operation of high-pressure gas discharge lamps is the lamp re-ignition after each zero passage of the mains alternating lamp current. More particularly, in metal halide discharge lamps, re-ignition voltages may be required during the heating-up stage that are higher than can be supplied by the ballast unit or the like, whereupon the lamp extinguishes. In order to facilitate the ignition and re-ignition, respectively, of high-pressure gas discharge lamps, an additional high frequency current has therefore been superimposed on lamps operated from a mains alternating voltage source.
In a circuit arrangement of this kind known from U.S. Pat. No. 4,378,514, in addition a high voltage having a frequency of 1.6 to 200 kHz is applied for igniting the lamps. This voltage is switched off again after ignition of the lamp. This high high-frequency voltage is higher than the ignition voltage of the lamps and could be at least 1000 V. The high-frequency oscillator has therefore to be constructed for such a voltage, which requires comparatively large high-power physical elements.
GB-PS No. 1,092,199 also discloses a circuit arrangement for A.C. operation of gas discharge lamps in which an additional high-frequency current is superimposed on the mains alternating lamp current, as a result of which the re-ignition voltage is reduced. The high-frequency superimposition takes place during the whole period duration of the mains alternating lamp current. The high-frequency current is about 10% of the average mains alternating lamp current. Thus, once again a comparatively large high-frequency oscillator is required.